An electrophotographic dry development method describes a method of development based on a powdered toner serving as a developer being affixed to an electrostatic latent image of a photosensitive material, and the affixed toner being transferred onto a predetermined paper or the like. Electrophotographic dry development methods may be divided into single-component development methods that employ a single component developer containing a toner alone, and two-component development methods that employ a two-component developer containing a toner and a magnetic electrophotographic developer carrier (hereinafter, also referred to as a magnetic carrier). Because of the stable high-image quality and capacity for high-speed development afforded by the simplification of toner charge control in recent years, two-component development methods are now widely employed.
While the trend in electrophotographic development apparatuses is toward apparatuses that enable full-color imaging and high-speed development with high-image quality, polymerized toners of small particle diameter have been developed as the toner employed to achieve the same, and development of magnetic carriers of small particle diameter and compatible with polymerized toners of small particle diameter is well under way. The market for so-called MFP (multi-function printer) electrophotographic development apparatuses has expanded accompanying the popularization of personal computers, and while simultaneously with these electrophotographic development apparatuses executing functions based on ancillary applications or the like, they are unfavorably appraised from the viewpoint of not only their document output capacity but also their running costs.
The running costs of an electrophotographic development apparatus are largely dependent on the cost of consumables such as the toner and magnetic carrier. Most magnetic carriers employ a spherical soft ferrite as an electrophotographic developer carrier core material (hereinafter also referred to as a carrier core material.) and, while a resin is coated on the surface of these spherical soft ferrites, the resin on the surface deteriorates as the print copy number increases due to abrasion caused by the magnetic carriers until a stage at which it is unfit for electrophotographic development is reached. For this reason, in most electrophotographic development apparatuses the magnetic carrier and toner are simultaneously replaced subsequent to a set value of the counted document print copy number being reached.
Patent Document 1 proposes a method of manufacturing a carrier core material of low density and low specific gravity in which, based on the use of a carbonate starting material as a carrier core material starting material and the utilization of the gasified component of this starting material, a hollow structure is generated in the carrier core material.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. S61-7851
The inventors of the present invention theorized the importance of reducing stress on the resin on the surface of the carrier core material for extending the replacement interval of a magnetic carrier. Furthermore, the inventors theorized that the stress that a carrier core material is subjected to when an electrophotographic developer is being agitated and mixed in an electrophotographic development apparatus can be reduced by reducing the specific gravity of the core material. It was apparent from examinations conducted by the inventors of the present invention that the manufacture of an electrophotographic developer employing a magnetic carrier manufactured by the method of manufacturing described in, for example, Patent Document 1, and the employment of this electrophotographic developer employed in an MFP or the like does not afford an extended magnetic carrier replacement interval.
Thereupon, the inventors of the present invention conducted further examinations as to the reasons preventing the replacement interval of a conventional magnetic carrier from being extended. The following was apparent as a result thereof. That is to say, while gasification of a carbonate starting material progresses when a carrier core material starting material is calcined and a hollow structure is formed in a calcined powder, this hollow structure is pulverized in a wet pulverization step implemented on this calcined power in which a hollow structure is formed in a ball mill that follows the calcination step. This is thought to be because, while a hollow structure is formed in a sintered powder generated in a subsequent sintering step as a result of the gasification of a residual portion of the carbonate starting material, the extent of this formation is restricted.
Furthermore, Patent Document 1 describes a configuration in which some of the carbonate starting material is apportioned for addition to the calcined starting material powder and sintered. However, it was apparent from examinations conducted by the inventors of the present invention that employment of the electrophotographic developer containing the magnetic carrier in which this configuration is employed in an above-noted MFP or the like does not afford an extended magnetic carrier replacement interval.
Thereupon, the inventors of the present invention conducted examinations as to the reasons preventing the replacement interval of this magnetic carrier from being extended. As a result, the reason preventing the magnetic carrier replacement interval of this configuration from being extended was thought to reside in an inadequate amount of gas being generated from the carbonate starting material and, as a natural outcome thereof, the formation of the hollow structure in the sintering step being restricted thereby.